1. Field of the Invention
The invention relates to a slit lens arrangement for particle beams, in particular for the projection of a mask onto a workpiece, comprising a combined lens, which consists of a cylinder lens and a quadrupole lens, the optical axes of which run parallel to each other, the optical axis of the quadrupole lens being displaceable in a parallel manner, the cylinder lens and the quadrupole lens in each case having a gap-like aperture between the pole shoes or in the electrodes with the same spatial orientation to one other, and the two lenses being arranged relative to each other, such that the focusing of the quadrupole lens occurs in that plane in which the cylinder lens is not focused, and the defocusing of the quadrupole lens occurs in that plane in which the cylinder lens focuses.
In the production of microscopically fine structures, optical and electron-optical processes are used. The latter process provides the advantage of increased resolution because of the substantially smaller wavelength of the electrons. This means that the distance between closely adjacent point of an object can take on smaller values in the electron-optical process before the two points in the image plane of the imaging arrangement are no longer separable as individual points. In the production of electronic devices and integrated circuits on the surface of disc-shaped semiconductor crystals (wafers), electron-optical lithographic processes therefore play an important role.
For electron-projection lithography, short focal length electron lenses are required, which image the greatest possible area of the mask on the wafer.
2. Description of the Prior Art
In the prior art, electron-optical circular lens systems are known, in which, by superimposition of a correction field on the circuit lens field, the optical axis can be displaced in a parallel manner in directions perpendicular to the optical axis. Here, the position of the optical axis determines the position of the writing spot on the wafer. With dynamic superimposition of the correction field, it is therefore possible to write on a planar wafer with this arrangement. As a disadvantage of this system, it has been found that the diameter of the area to be written on is very much smaller than the bore radius of the lens pole shoe. The consequence of this is that, for writing on workpieces with extended dimensions, for example a wafer, this present arrangement must be equipped with a slide, which receives the workpiece and is displaceable perpendicular to the optical axis in two coordinate directions. The carriage, which is movable in two dimensions, with the required high precision, however is very complicated to design and is correspondingly expensive to produce. Also, because of the comparatively low displacement rate and the additional checking of the positioning, it limits the efficiency of the device.
In addition, two devices of different design for electron beam lithography are known, in which electron-optical elements with non-circular fields are employed.
The first device of this kind is disclosed in DE 196 34 456. It comprises an electrostatic cylinder lens and a magnetic quadrupole, which in combination form an image stigmatically in the same way as a circular lens. On the quadrupole, a magnetic dipole field can be superimposed, which effects a displacement of the optical axis in the direction of the longitudinal extension of the cylinder lens gap, the position of the optical axis again determining the position of the write dot on the wafer. The aforementioned combined lens retains its imaging properties during displacement of the optical axis over practically the entire length of the cylinder lens gap. Correspondingly, with the present non-circular system, compared with the aforementioned circular lens, substantially larger areas of the wafer can be written on without it being mechanically displaced. The disadvantage of the present device can be seen in the fact that the wafer must be written on point-by-point by the focused electron beam, and therefore the parallel projection of an extended mask region, as is conventional with electron projection lithography, is not possible.
The second device of the aforementioned type is disclosed in DE 196 44 857. It comprises an electrostatic cylinder lens consisting of at least three electrodes, of which the centre electrode in the longitudinal direction of the gap-like electrode aperture comprises segments insulated electrically from one another (comb electrodes). By virtue of this construction, different potentials can be applied to the individual segments of the centre electrode. In the present device, the potential distribution is chosen such that an electrostatic quadrupole is generated. Like the aforementioned device, the present arrangement thus also comprises a cylinder lens combined with a quadrupole lens, but with the difference that the quadrupole is electrical in nature. The imaging properties of the present lens system, in a disadvantageous manner, largely correspond to those of the aforementioned system.